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Thread: Need help w/ PSU for a Compaq SystemPro XL

  1. #1

    Question Need help w/ PSU for a Compaq SystemPro XL

    I have a very clean and well stocked Compaq SystemPro XL (w/ original box and paper work) thanks to Chulofiasco.

    Unfortunately, the PSU died while CF was setting it up and it remains so. I am very sure it is a PSU problem. On switching the machine "on" the power comes on for a second (lights shine, fan spins) then it shuts down. Three seconds later it does it again. According to the service manual for the original CompaqSystem Pro (the only service manual I have access to) this is because of a over voltage/under voltage issue and is a safety mechanism. I have tried trouble shooting the PSU with Druid6900 but between my lack of skills and a lack of proper documentation (e.g. schematics) we have not gotten very far. Being a Compaq PSU of course it uses non-standard connectors so finding what is going on is a bit more complicated. I also found another PSU online which I bought. Unfortunately, this one is even in worse shape. Powering it on only produces a loud buzzing sound.

    I really would like to get this system up and running. Does anyone have any experience with this PSU or better yet have access to schematics and/or the service manual for the SystemPro XL? Barring direct information anyone have advice?

    Pictures of innards are forthcoming....

    TIA!
    Current Wish List: 1. IBM 7531 Industrial Series PC 2. NEC MultiSync XL (JC-2001) Monitor 3. Copy II PC Enhanced Option Board (the one with the toggle switch on the back) 4. MicroSolutions MatchPoint AND/OR UniDOS card 5. Compaq 14" VGA CRT Monitor (the one that came with the SystemPro). If you have any of the above for sale please PM me. Thank you!

  2. #2
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    Are you running with the PSU attached to the system or standalone?

  3. #3

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    Quote Originally Posted by GiGaBiTe View Post
    Are you running with the PSU attached to the system or standalone?
    Not sure what you mean. For the initial attempt to start up the psu was attached to the system with multiple drives for load.
    Current Wish List: 1. IBM 7531 Industrial Series PC 2. NEC MultiSync XL (JC-2001) Monitor 3. Copy II PC Enhanced Option Board (the one with the toggle switch on the back) 4. MicroSolutions MatchPoint AND/OR UniDOS card 5. Compaq 14" VGA CRT Monitor (the one that came with the SystemPro). If you have any of the above for sale please PM me. Thank you!

  4. #4
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    You aren't sure if the PSU is attached to something or not? Not that hard of a question.

    Having it attached to a bunch of stuff is too many variables, you need to start it without any load to see if it stays running. Power supplies by that time were advanced enough to not need a load to remain on and be mostly within regulation.

    If there's an issue with the supply, you most definitely do not want to be attaching it to any equipment, especially if it keeps power cycling every few seconds. That will rapidly destroy hard drives and is very hard on other components.

    Switching power supplies most often shut off because of an overload (usually a short to ground) and less often shut off because of an over/under voltage on a rail. That wasn't a common power supply protection until over a decade later, but I won't rule it out existing on that unit. Try powering on the power supply while not attached to anything in the system. If it turns on, then there's something wrong with the system, if not then it's the supply. The supply that makes a loud buzzing noise likely has bad line capacitors in it, when line caps go bad it'll make the primary switching mosfets go haywire, a sign is the coils the mosfets power making chirping/buzzing noises.

    I suggest you remove the power supply from the machine and start hunting for shorts on the power rails on the motherboard. Use a multimeter in ohms mode between ground and each individual power rail to see if you get a reading of less than 10k ohm. If you get a reading that's only a few ohms or a few hundred ohms, there's a short somewhere that needs to be fixed. If you have cards installed, remove those one at a time to see if the short goes away. If it does, it's on one of the cards, if it doesn't, it's on the motherboard. If you can't find any shorts, check the hard drives and any other peripherals attached to the supply.

  5. #5

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    Quote Originally Posted by GiGaBiTe View Post
    You aren't sure if the PSU is attached to something or not? Not that hard of a question.
    No, my original message clearly states lights came on and fans turned indicating the PSU was connected to the computer when I tried turning it on so I was not sure why you were asking if it was connected to anything...

    Quote Originally Posted by GiGaBiTe View Post
    Having it attached to a bunch of stuff is too many variables, you need to start it without any load to see if it stays running. Power supplies by that time were advanced enough to not need a load to remain on and be mostly within regulation.
    Did that already with Druid6900 and the PSU does nothing when it is not attached. If the SM is to be believed (since it is for an earlier model) load determines how fast the PSU will reach its minimal output voltage. To my uneducated ears that sounds like if you have a load on the PSU will reach steady state faster but I could be wrong. In either case, if the PSU is not connected nothing happens.

    Quote Originally Posted by GiGaBiTe View Post
    Switching power supplies most often shut off because of an overload (usually a short to ground) and less often shut off because of an over/under voltage on a rail. That wasn't a common power supply
    protection until over a decade later, but I won't rule it out existing on that unit.
    Not sure when that protection went mainstream but the SM indicates again the earlier PSU from 89 had it so I am assuming the one from 92 does as well...

    Quote Originally Posted by GiGaBiTe View Post
    The supply that makes a loud buzzing noise likely has bad line capacitors in it, when line caps go bad it'll make the primary switching mosfets go haywire, a sign is the coils the mosfets power making chirping/buzzing noises.
    Well caps are easy enough to switch. I have yet to open that particular PSU but the other one does not have any obviously bad caps.

    Quote Originally Posted by GiGaBiTe View Post
    I suggest you remove the power supply from the machine and start hunting for shorts on the power rails on the motherboard. Use a multimeter in ohms mode between ground and each individual power rail to see if you get a reading of less than 10k ohm. If you get a reading that's only a few ohms or a few hundred ohms, there's a short somewhere that needs to be fixed.
    So this would be done on an unpowered system, correct?
    Current Wish List: 1. IBM 7531 Industrial Series PC 2. NEC MultiSync XL (JC-2001) Monitor 3. Copy II PC Enhanced Option Board (the one with the toggle switch on the back) 4. MicroSolutions MatchPoint AND/OR UniDOS card 5. Compaq 14" VGA CRT Monitor (the one that came with the SystemPro). If you have any of the above for sale please PM me. Thank you!

  6. #6

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    As a follow up question why would a functioning system (with the same current config) suddenly develop a short? i.e. asked another way what kind of component failing would make a system in a steady state suddenly develop a short?
    Current Wish List: 1. IBM 7531 Industrial Series PC 2. NEC MultiSync XL (JC-2001) Monitor 3. Copy II PC Enhanced Option Board (the one with the toggle switch on the back) 4. MicroSolutions MatchPoint AND/OR UniDOS card 5. Compaq 14" VGA CRT Monitor (the one that came with the SystemPro). If you have any of the above for sale please PM me. Thank you!

  7. #7

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    classic failure mode for tantalum capacitors.

  8. #8

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    Quote Originally Posted by bear View Post
    classic failure mode for tantalum capacitors.
    Bear,

    That was my initial thinking as well. However, all the caps look pretty good. The PSU is actually in very good VISUAL shape for its age. I will post pictures later today. I have access to an ESR meter so I can ceck the smaller capacitors.
    Current Wish List: 1. IBM 7531 Industrial Series PC 2. NEC MultiSync XL (JC-2001) Monitor 3. Copy II PC Enhanced Option Board (the one with the toggle switch on the back) 4. MicroSolutions MatchPoint AND/OR UniDOS card 5. Compaq 14" VGA CRT Monitor (the one that came with the SystemPro). If you have any of the above for sale please PM me. Thank you!

  9. #9

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    Images as promised:









    And the non-std. MB connector (which is different from the connector they used on the original SystemPro PSU)

    Current Wish List: 1. IBM 7531 Industrial Series PC 2. NEC MultiSync XL (JC-2001) Monitor 3. Copy II PC Enhanced Option Board (the one with the toggle switch on the back) 4. MicroSolutions MatchPoint AND/OR UniDOS card 5. Compaq 14" VGA CRT Monitor (the one that came with the SystemPro). If you have any of the above for sale please PM me. Thank you!

  10. #10

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    sorry for the small image sizes. I went to the trouble of uploading them to an image service so that the full res 5MP image would be linked but apparently the image service decided to shrink them anyhow. If anyone has a recommendation for a good service that is free and handles bigger sizes (ideally without needing an account) please let me know! TIA!
    Current Wish List: 1. IBM 7531 Industrial Series PC 2. NEC MultiSync XL (JC-2001) Monitor 3. Copy II PC Enhanced Option Board (the one with the toggle switch on the back) 4. MicroSolutions MatchPoint AND/OR UniDOS card 5. Compaq 14" VGA CRT Monitor (the one that came with the SystemPro). If you have any of the above for sale please PM me. Thank you!

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